1. Field of the Invention
The present invention relates generally to air supply control systems for internal combustion engines, and more particularly to a system for controlling air supply effected to an internal combustion engine by a plurality of superchargers accompanying with the internal combustion engine.
2. Description of the Prior Art
In the field of internal combustion engines used in vehicles, there has been proposed a so-called sequentially controlled supercharging system in which a couple of primary and secondary turbosuperchargers are employed in an internal combustion engine and so controlled that only the primary turbosupercharger works for supercharging the engine when intake air mass flow in an intake passage of the engine is relatively small and both the primary and the secondary turbosuperchargers work simultaneously for supercharging the engine when the intake air mass flow is relatively large, as disclosed in, for example, the Japanese utility model application published after examination under publication number 57-12177 and the Japanese patent application published before examination under publication number 60-259722. In such a system, an exhaust cutoff valve is disposed in a portion of an exhaust passage of the engine through which exhaust gas is applied to a turbine of the secondary turbosupercharger and an intake air cutoff valve is also disposed in a portion of the intake passage of the engine through which air compressed by a blower of the secondary turbosupercharger is supplied to a combustion chamber of the engine, and each of the exhaust cutoff valve and the intake cutoff valve is controlled to be close and open so as to cause the primary and secondary turbosuperchargers to operate in the aforementioned manner.
In connection with the control in operation of the primary and secondary turbosuperchargers, it has been proposed to put the secondary turbosupercharger in a condition of preliminary rotation before the secondary turbosupercharger commences to work for supercharging the engine for the purpose of suppressing torque shock arising on the engine due to time lag in the starting characteristic of the secondary turbosupercharger. In such a case, it has been usual that the preliminary rotation of the secondary turbosupercharger is caused by such a control as to open the exhaust cutoff valve for supplying the turbine of the secondary turbosupercharger with the exhaust gas under a condition in which a intake air relief valve which is provided in a bypass provided to the intake passage for detouring the secondary turbosupercharger is opened.
However, the preliminary rotation of the secondary turbosupercharger thus caused in the manner proposed previously brings about a disadvantage that the primary turbosupercharger is undesirably reduced in its speed of rotation because the exhaust gas branches through the exhaust cutoff valve which is opened under the condition in which the intake air relief valve is opened and thereby the exhaust gas supplied to a turbine of the primary turbosupercharger is reduced and this compels the preliminary rotation of the secondary turbosupercharger to be conducted insufficiently for a relatively short period of time just before the secondary turbosupercharger commences to work for supercharging the engine. Accordingly, in the case where the preliminary rotation of the secondary turbosupercharger is conducted in the manner proposed previously, it is difficult to put the secondary turbosupercharger in a condition of sufficiently high preliminary rotation before the secondary turbosupercharger commences to work for supercharging the engine, and therefore the torque shock arising on the engine is not sufficiently reduced when the secondary turbosupercharger commences to work for supercharging the engine.